Thin film transistors (TFTs) are a type of field effect transistors (hereinafter referred to as FETs). TFTs are three-terminal elements having a gate terminal, a source terminal, and a drain terminal in the basic structure. TFTs are active elements having a function of switching the current between the source terminal and the drain terminal so that a semiconductor thin film deposited on a substrate is used as a channel layer in which electrons or holes move and a voltage is applied to the gate terminal to control the current flowing in the channel layer. TFTs are electronic devices that are most widely used these days in practical application. Typical applications of TFTs include liquid-crystal driving elements.
Currently, most widely used TFTs are metal-insulator-semiconductor-FETs (MIS-FETs) in which a polycrystalline silicon film or an amorphous silicon film is used as a channel layer material.
MIS-FETs including silicon are opaque to visible light and thus fail to form transparent circuits. Therefore, when MIS-FETs are used as switching elements for driving liquid crystals in liquid crystal displays, the aperture ratio of a display pixel in the devices is small.
Due to the recent need for high-resolution liquid crystals, switching elements for driving liquid crystals now require high-speed driving. In order to achieve high-speed driving, a semiconductor thin film in which the mobility of electrons or holes, is higher than that in at least amorphous silicon needs to be used as a channel layer.
Under such circumstances, Patent Document 1 proposes a transparent semi-insulating amorphous oxide thin film which is a transparent amorphous oxide thin film deposited by vapor-phase film deposition method and containing elements of In, Ga, Zn, and O. The composition of the oxide is InGaO3(ZnO)m, (m is a natural number less than 6) when the oxide is crystallized. The transparent semi-insulating amorphous oxide thin film is a semi-insulating thin film having a carrier mobility (also referred to as carrier electron mobility) of more than 1 cm2 V−1 sec−1, and a carrier density (also referred to as carrier electron density) of 1016 cm−3 or less without doping with an impurity ion. Patent Document 1 also proposes a thin film transistor in which the transparent semi-insulating amorphous oxide thin film is used as a channel layer.
However, the transparent amorphous oxide thin film (a-IGZO film) which is proposed in Patent Document 1, formed by either a vapor deposition method of a sputtering method or a pulsed laser deposition method, and composed of elements of In, Ga, Zn, and O has a relatively high electron carrier mobility in a range of about from 1 to 10 cm2 V−1 sec−1, but the instability has been often pointed out as a problem when forming a device such as a TFT as the fact that the amorphous oxide thin film is originally likely to generate oxygen loss and the behavior of the electron carrier is not always stable against external factors such as heat caused adverse effects.
As a material to solve such a problem, Patent Document 2 proposes a thin film transistor including an oxide thin film in which gallium is dissolved in indium oxide. In the oxide thin film, the Ga/(Ga+In) atomic ratio is 0.001 to 0.12, and the percentage of indium and gallium with respect to the total metal atoms is 80 at % or more. The oxide thin film has an In2O3 bixbyite structure. An oxide sintered body is proposed as the material of the oxide thin film in which gallium is dissolved in indium oxide. In the oxide sintered body, the Ga/(Ga+In) atomic ratio is 0.001 to 0.12, and the percentage of indium and gallium with respect to the total metal atoms is 80 at % or more. The oxide sintered body has an In2O3 bixbyite structure.
However, the carrier density described in Examples 1 to 8 of Patent Document 2 is at the level of 1018 cm−3, and there is still a problem that the carrier density is too high for an oxide semiconductor thin film to be applied to a TFT.
In addition, in Patent Document 3, a sintered body in which the gallium content ratio and the copper content ratio in indium, gallium, and copper are more than 0.001 and less than 0.09 in terms of atomic ratio is disclosed.
However, the sintered body disclosed in Patent Document 3 is substantially an In2O3 phase having a bixbyite-type structure or an In2O3 phase having a bixbyite-type structure and an In2Ga2CuO7 phase having a hexagonal structure and/or an InGaCuO4 phase have having a rhombohedral structure, but the density of the sintered body is low because the sintering temperature is from 1000° C. to 1100° C. and phases having a relatively high electrical resistance are formed other than the In2O3 phase, and thus there is a problem in that nodules are likely to be generated in the mass-production process of film deposition by sputtering to introduce a high-power density.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2010-219538
Patent Document 2: PCT International Publication No. WO2010/032422
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2012-012659
Patent Document 4: PCT International Publication No. WO2003/014409
Patent Document 5: Japanese Unexamined Patent Application, Publication No. 2012-253372